The Cryptococcus neoformans STE11alpha gene is similar to other fungal mitogen-activated protein kinase kinase kinase (MAPKKK) genes but is mating type specific

Partial sequence analysis of the Cryptococcus neoformans MATalpha mating type locus revealed the presence of a gene with substantial sequence similarity to other fungal mitogen-activated protein (MAP) kinase kinase kinase (MAPKKK) genes. The C. neoformans gene, designated STE11alpha, showed the highest degree of similarity to the Neurospora crassa nrc-1, Schizosaccharomyces pombe byr2 and Saccharomyces cerevisiae STE11 genes. A polymerase chain reaction-mediated sib-selection technique was successfully adapted for the purpose of disrupting STE11alpha. C. neoformans ste11alphaDelta mutants were found to be sterile, consistent with the phenotypes of ste11 and byr2 mutants in S. cerevisiae and S. pombe respectively. Haploid ste11alphaDelta mutants were also found to be unable to produce hyphae, suggesting that the C. neoformans gene is functionally conserved when compared with its S. cerevisiae MAPKKK counterpart. Comparison of the wild-type STE11alpha strain with a ste11alphaDelta disruptant for virulence using the mouse model showed that the ste11alphaDelta strain was less virulent, but the difference was only minor. In spite of some of the conserved functions of STE11alpha, linkage analysis showed that STE11alpha is only found in mating type alpha strains. These results demonstrate that, although functionally conserved, the mating pathway in C. neoformans has a unique organization.     

Tagging morphogenetic genes by insertional mutagenesis in the yeast Yarrowia lipolytica

The yeast Yarrowia lipolytica is distantly related to Saccharomyces cerevisiae, can be genetically modified, and can grow in both haploid and diploid states in either yeast, pseudomycelial, or mycelial forms, depending on environmental conditions. Previous results have indicated that the STE and RIM pathways, which mediate cellular switching in other dimorphic yeasts, are not required for Y. lipolytica morphogenesis. To identify the pathways involved in morphogenesis, we mutagenized a wild-type strain of Y. lipolytica with a Tn3 derivative. We isolated eight tagged mutants, entirely defective in hyphal formation, from a total of 40,000 mutants and identified seven genes homologous to S. cerevisiae CDC25, RAS2, BUD6, KEX2, GPI7, SNF5, and PPH21. We analyzed their abilities to invade agar and to form pseudomycelium or hyphae under inducing conditions and their sensitivity to temperature and to Calcofluor white. Chitin staining was used to detect defects in their cell walls. Our results indicate that a functional Ras-cyclic AMP pathway is required for the formation of hyphae in Y. lipolytica and that perturbations in the processing of extracellular, possibly parietal, proteins result in morphogenetic defects.     

The isolation and characterization of the pyruvate kinase-encoding gene from the yeast Yarrowia lipolytica.

The dimorphic yeast, Yarrowia lipolytica, has been developed as a useful expression/secretion system for heterologous proteins such as chymosin and tissue plasminogen activator. To further develop this expression system, we have cloned the gene (PYK) encoding the highly expressed glycolytic enzyme, pyruvate kinase (PYK). Genomic clones were selected by their specific hybridization to synthetic oligodeoxyribonucleotide probes based on regions of the enzyme that were conserved through evolution. The clones identified by hybridization contained overlapping DNA inserts. We have confirmed the identity of the cloned gene based on two criteria: (1) the nucleotide sequence of the proposed PYK gene predicts a protein that is highly homologous to the corresponding Saccharomyces cerevisiae enzyme, and (2) PYK-specific activity was increased twofold when wild-type Y. lipolytica strains were transformed with the isolated DNA. Interestingly, we found that the open reading frame of the Y. lipolytica PYK gene was interrupted by an intron. This represents the first report of an intron in a Y. lipolytica gene.     

Protein kinases involved in mating and osmotic stress in the yeast Kluyveromyces lactis

Systematic disruption of genes encoding kinases and mitogen-activated protein kinases (MAPKs) was performed in Kluyveromyces lactis haploid cells. The mutated strains were assayed by their capacity to mate and to respond to hyperosmotic stress. The K. lactis Ste11p (KlSte11p) MAPK kinase kinase (MAPKKK) was found to act in both mating and osmoresponse pathways while the scaffold KlSte5p and the MAPK KlFus3p appeared to be specific for mating. The p21-activated kinase KlSte20p and the kinase KlSte50p participated in both pathways. Protein association experiments showed interaction of KlSte50p and KlSte20p with Gα and Gβ, respectively, the G protein subunits involved in the mating pathway. Both KlSte50p and KlSte20p also showed interaction with KlSte11p. Disruption mutants of the K. lactis PBS2 (KlPBS2) and KlHOG1 genes of the canonical osmotic response pathway resulted in mutations sensitive to high salt and high sorbitol but dispensable for mating. Mutations that eliminate the MAPKK KlSte7p activity had a strong effect on mating and also showed sensitivity to osmotic stress. Finally, we found evidence of physical interaction between KlSte7p and KlHog1p, in addition to diminished Hog1p phosphorylation after a hyperosmotic shock in cells lacking KlSte7p. This study reveals novel roles for components of transduction systems in yeast.     

Ste50 adaptor protein governs sexual differentiation of Cryptococcus neoformans via the pheromone-response MAPK signaling pathway

The mitogen-activated protein kinase (MAPK) pathways control diverse cellular functions in pathogenic fungi, including sexual differentiation, stress response, and maintenance of cell wall integrity. Here we characterized a Cryptococcus neoformans gene, which is homologous to the yeast Ste50 that is known to play an important role in mating pheromone response and stress response as an adaptor protein to the Ste11 MAPK kinase kinase in Saccharomyces cerevisiae. The C. neoformans Ste50 was not involved in any of the stress responses or virulence factor production (capsule and melanin) that are controlled by the HOG and Ras/cAMP signaling pathways. However, Ste50 was required for mating in both serotype A and serotype D C. neoformans strains. The ste50Δ mutant was completely defective in cell-cell fusion and mating pheromone production. Double mutation of the STE50 gene blocked increased production of pheromone and the hyper-filamentation phenotype of cells deleted of the CRG1 gene, which encodes the RGS protein that negatively regulates pheromone responsive G-protein signaling via the MAPK pathway. Regardless of the presence of the basidiomycota-specific SH3 domains of Ste50 that are known to be required for full virulence of Ustilago maydis, Ste50 was dispensable for virulence of C. neoformans in a murine model of cryptococcosis. In conclusion, the Ste50 adaptor protein controls sexual differentiation of C. neoformans via the pheromone-responsive MAPK pathway but is not required for virulence.     

Mating-defective ste mutations are suppressed by cell division cycle start mutations in Saccharomyces cerevisiae.

Temperature-sensitive mutants which arrest in the G1 phase of the cell cycle have been described for the yeast Saccharomyces cerevisiae. One class of these mutants (carrying cdc28, cdc36, cdc37, or cdc39) forms a shmoo morphology at restrictive temperature, characteristic of mating pheromone-arrested wild-type cells. Therefore, one hypothesis to explain the control of cell division by mating factors states that mating pheromones arrest wild-type cells by inactivating one or more of these CDC gene products. A class of mutants (carrying ste4, ste5, ste7, ste11, or ste12) which is insensitive to mating pheromone and sterile has also been described. One possible function of the STE gene products is the inactivation of the CDC gene products in the presence of a mating pheromone. A model incorporating these two hypotheses predicts that such STE gene products will not be required for mating in strains carrying an appropriate cdc lesion. This prediction was tested by assaying the mating abilities of double mutants for all of the pairwise combinations of cdc and ste mutations. Lesions in either cdc36 or cdc39 suppressed the mating defect due to ste4 and ste5. Allele specificity was observed in the suppression of both ste4 and ste5. The results indicate that the CDC36, CDC39, STE4, and STE5 gene products interact functionally or physically or both in the regulation of cell division mediated by the presence or absence of mating pheromones. The cdc36 and cdc39 mutations did not suppress ste7, ste11, or ste12. Lesions in cdc28 or cdc37 did not suppress any of the ste mutations. Other models of CDC and STE gene action which predicted that some of the cdc and ste mutations would be alleles of the same locus were tested. None of the cdc mutations was allelic to the ste mutations and, therefore, these models were eliminated.     

Yarrowia lipolytica mutants devoid of pyruvate carboxylase activity show an unusual growth phenotype

We have cloned and characterized the gene PYC1, encoding the unique pyruvate carboxylase in the dimorphic yeast Yarrowia lipolytica. The protein putatively encoded by the cDNA has a length of 1,192 amino acids and shows around 70% identity with pyruvate carboxylases from other organisms. The corresponding genomic DNA possesses an intron of 269 bp located 133 bp downstream of the starting ATG. In the branch motif of the intron, the sequence CCCTAAC, not previously found at this place in spliceosomal introns of Y. lipolytica, was uncovered. Disruption of the PYC1 gene from Y. lipolytica did not abolish growth in glucose-ammonium medium, as is the case in other eukaryotic microorganisms. This unusual growth phenotype was due to an incomplete glucose repression of the function of the glyoxylate cycle, as shown by the lack of growth in that medium of double pyc1 icl1 mutants lacking both pyruvate carboxylase and isocitrate lyase activity. These mutants grew when glutamate, aspartate, or Casamino Acids were added to the glucose-ammonium medium. The cDNA from the Y. lipolytica PYC1 gene complemented the growth defect of a Saccharomyces cerevisiae pyc1 pyc2 mutant, but introduction of either the S. cerevisiae PYC1 or PYC2 gene into Y. lipolytica did not result in detectable pyruvate carboxylase activity or in growth on glucose-ammonium of a Y. lipolytica pyc1 icl1 double mutant.     

Serum-induced hypha formation in the dimorphic yeast Yarrowia lipolytica

The dimorphic yeast Yarrowia lipolytica forms true hyphae in a medium containing N-acetylglucosamine. We made a new finding that serum is a very effective inducer of hypha formation of Y. lipolytica: serum induced its hyphal growth very quickly compared to N-acetylglucosamine (4 h vs. 10 h). Osmotic and oxidative stresses (0.2 M NaCl and 20 mM H2O2) inhibited the hypha formation induced by N-acetylglucosamine, but did not suppress the hypha formation triggered by serum. Serum-specific morphological mutants, which formed hyphae in the N-acetylglucosamine medium but not in serum medium, could be isolated. These results suggest that the signal triggered by serum may be transduced through a different pathway, at least in part, from that used for the N-acetylglucosamine signal in Y. lipolytica.     

The regulatory subunit of protein kinase A promotes hyphal growth and plays an essential role in Yarrowia lipolytica

The gene encoding the regulatory subunit (RKA1) of the cAMP-dependent protein kinase (PKA) of Yarrowia lipolytica was isolated to analyze the role of the PKA pathway in the dimorphic transition of the fungus. The gene encoded a protein of 397 amino acids that exhibits significant homology to fungal PKA regulatory subunits. Attempts to disrupt the gene by double homologous recombination, or the Pop-in Pop-out technique, were unsuccessful. The gene could be mutated only in merodiploids constructed with an autonomous replicating plasmid. Loss of the plasmid occurred with growth under nonselective conditions in the whole population of merodiploids carrying the mutation in the plasmid, but in merodiploids with the mutation at the chromosome, a resistant population prevailed. These data suggest that RKA1 is essential in Y. lipolytica. cAMP addition inhibited the dimorphic transition of the parental strain, but merodiploids carrying several copies of RKA1 were more resistant to cAMP. These results, and the observation that RKA1 was upregulated in mycelial cells, indicate that an active PKA pathway promotes yeast-like growth and opposes mycelial development. This behavior is in contrast to that of Candida albicans, where the PKA pathway favors hyphal growth.     

Differential transmission of G1 cell cycle arrest and mating signals by Saccharomyces cerevisiae Ste5 mutants in the pheromone pathway.

Saccharomyces cerevisiae Ste5 is a scaffold protein that recruits many pheromone signaling molecules to sequester the pheromone pathway from other homologous mitogen-activated protein kinase pathways. G1 cell cycle arrest and mating are two different physiological consequences of pheromone signal transduction and Ste5 is required for both processes. However, the roles of Ste5 in G1 arrest and mating are not fully understood. To understand the roles of Ste5 better, we isolated 150 G1 cell cycle arrest defective STE5 mutants by chemical mutagenesis of the gene. Here, we found that two G1 cell cycle arrest defective STE5 mutants (ste5M(D248V) and ste5(delta-776)) retained mating capacity. When overproduced in a wild-type strain, several ste5 mutants also showed different dominant phenotypes for G1 arrest and mating. Isolation and characterization of the mutants suggested separable roles of Ste5 in G1 arrest and mating of S. cerevisiae. In addition, the roles of Asp-248 and Tyr-421, which are important for pheromone signal transduction were further characterized by site-directed mutagenesis studies.     

The Ambient pH Response Rim Pathway in Yarrowia lipolytica: Identification of YlRIM9 and Characterization of Its Role in Dimorphism

Yarrowia lipolytica is a dimorphic fungus that secretes either an acidic or an alkaline protease depending on the environmental pH. Previous results have indicated that secretion of the alkaline protease is under control of the pH signaling Pal/Rim pathway originally described in Aspergillus nidulans. Several Y. lipolytica mutants defective in some Rim components of this pathway have been previously isolated and the RIM genes characterized. In the present study, Y. lipolytica RIM9 (palI) gene (YlRIM9) was sequenced from a plasmid (AL414126) of the Genolevures project (the DNA sequence data for YlRIM9 gene has been deposited at EMBL with accession number AJ566902). The derived translation product contains 724 amino acids with a predicted signal peptide and four transmembrane domains in its N-terminal region. We demonstrated that mutation in YlRIM9, as well as in other genes encoding members of the Pal/Rim pathway, did not affect the pH-dependent dimorphic transition of Y. lipolytica. A different pathway must exist in this fungus that controls the effect of pH on dimorphism.     

An ste20 homologue in Ustilago maydis plays a role in mating and pathogenicity

The mitogen-activated protein kinase (MAPK) pathways are conserved from fungi to humans and have been shown to play important roles in mating and filamentous growth for both Saccharomyces cerevisiae and dimorphic fungi and in infectivity for pathogenic fungi. STE20 encodes a protein kinase of the p21-activated protein kinase family that regulates more than one of these cascades in yeasts. We hypothesized that an Ste20p homologue would play a similar role in the dimorphic plant pathogen Ustilago maydis. The full-length copy of the U. maydis gene was obtained from a genomic library; it lacked introns and was predicted to encode a protein of 826 amino acids, whose sequence confirmed its identity as the first Ste20p homologue to be isolated from a plant pathogen. The predicted protein contained both an N-terminal regulatory Cdc42-Rac interactive binding domain and a C-terminal catalytic kinase domain. Disruption of the gene smu1 resulted in a delayed mating response in a mating-type-specific manner and also in a severe reduction in disease production on maize. Unlike the Ustilago bypass of cyclase (ubc) mutations previously identified in genes in the pheromone-responsive MAPK cascade, mutation of smu1 does not by itself act as an extragenic suppressor of the filamentous phenotype of a uac1 mutant. Thus, the direct connection of Smu1p to MAPK cascade function has yet to be established. Even so, Smu1, though not absolutely required for mating, is necessary for wild-type mating and pathogenicity.     

Adaptor functions of Cdc42, Ste50, and Sho1 in the yeast osmoregulatory HOG MAPK pathway

The yeast high osmolarity glycerol (HOG) signaling pathway can be activated by either of the two upstream pathways, termed the SHO1 and SLN1 branches. When stimulated by high osmolarity, the SHO1 branch activates an MAP kinase module composed of the Ste11 MAPKKK, the Pbs2 MAPKK, and the Hog1 MAPK. To investigate how osmostress activates this MAPK module, we isolated both gain-of-function and loss-of-function alleles in four key genes involved in the SHO1 branch, namely SHO1, CDC42, STE50, and STE11. These mutants were characterized using an HOG-dependent reporter gene, 8xCRE-lacZ. We found that Cdc42, in addition to binding and activating the PAK-like kinases Ste20 and Cla4, binds to the Ste11-Ste50 complex to bring activated Ste20/Cla4 to their substrate Ste11. Activated Ste11 and its HOG pathway-specific substrate, Pbs2, are brought together by Sho1; the Ste11-Ste50 complex binds to the cytoplasmic domain of Sho1, to which Pbs2 also binds. Thus, Cdc42, Ste50, and Sho1 act as adaptor proteins that control the flow of the osmostress signal from Ste20/Cla4 to Ste11, then to Pbs2.     

A novel MAP-kinase kinase from Candida albicans

A putative MAP-kinase kinase-encoding gene, CaSTE7, was isolated from Candida albicans by complementation of ste7 and stell mutants of the pheromone signal-transduction pathway of Saccharomyces cerevisiae. The nucleotide (nt) sequence revealed an ORF of 1767 nt encoding a putative protein of 589 amino acids (aa). CaSTE7 has a strong homology with MAP-kinase kinase STE7 of S. cerevisiae, the kinase domain having 45% homology with that of STE7. The deduced aa sequence contained all eleven consensus kinase subdomains found in MAP-kinase kinases. It can suppress the mating defect of ste5, stell, ste7, and fus3 kssl double mutants, but it cannot bypass the ste12 mutation. CaSTE7 behaves as a hyperactive allele of STE7, suppressing the mating defects of the pheromone signal-transduction pathway by constitutively stimulating STE12, and hence STE12-dependent processes.